This invention relates to plastic film laminators of the type used for applying protective coverings to drivers licenses, identification cards, menus, photographs and other products formed of laminated plastic film. Such products typically comprise a paper card or the like placed between layers of plastic film such as polyethylene-coated Mylar. The plastic layers extend somewhat beyond the edges of the card. When the edges of the plastic layers are sealed by heating, a protective pocket or pouch is formed around the card.
Generally, there are two main types of laminating machines. In smaller machines, heating of the plastic film to aid in lamination is accomplished by means of flat platens. Expeller rollers downstream of the platens compress the heated film. An early example of a flat platen laminator is Elliott U.S. Pat. No. 2,927,620. A more recent example is Levitan U.S. Pat. No. 3,770,550. The other basic type of laminating machine uses heated rollers in the lamination process. These are usually larger machines. An example is Eisner U.S. Pat. No. 3,027,285.
An important application for pouch laminating machines is laminating substrates which are impervious to air. Such substrates include glossy photographs and inserts constructed of die-cut polymer sheets. A major drawback of current flat platen laminating machines is their tendency to entrap air bubbles or moisture between the laminate film and impervious substrates. This detracts from the picture quality, and can render data encoding schemes such as magnetic stripes and bar codes unreliable.
The cause of entrapment in flat platen laminators is as follows. The flat, heated platens are in contact with the film to heat and melt the adhesive. Often the platens are in pressure contact with the laminate workpiece, and the pressure can cause pre-lamination of the plastic and adhesive to the substrate prior to reaching the downstream expeller rollers. Once pre-lamination has occurred in the platens, the cold expeller rollers cannot undo the pre-lamination and the bubbles are trapped.
Heated roller machines are not as prone to entrapment as flat platen machines but they suffer from productivity problems. These machines use roller heat alone to melt the adhesive layer. Because the roller nip is relatively narrow and the dwell time is very short, the feed rate of the workpiece must be slow or the rollers must operate at an extremely high temperature. Otherwise insufficient heat is transferred to the workpiece to effect complete lamination. It is preferable to avoid the slow feed rates and high operating temperatures of heated roller laminators, and also prevent the entrapment problems of heated platen laminators. The present invention makes this possible.